1. Field of Invention
The invention relates to systems and methods for deskewing a substrate.
2. Description of Related Art
There are a variety of transport and deskew systems in use that transport and register various substrates, such as copy sheets. In many deskew systems, such as those often found in copiers, facsimiles, and printers (i.e., reproduction systems), drive mechanisms often include at least one driven elastomer-covered roll backed by a hard idler roll to form a roll pair defining a nip region there between. A substrate, such as copy paper, provided to the nip region is advanced by rotation of the roll pair, specifically rotation of the driven roll, which causes corresponding linear movement of the substrate, such as paper.
Paper skew is the angular deviation of the longitudinal axis of the substrate in the process direction and/or the angular deviation of the lateral axis of the substrate perpendicular to the process direction. The lateral edges are the edges of the sheets that are substantially parallel to the process direction. The process edges are edges of the sheets that are substantially perpendicular to the process direction. The process edges may be referred to as the leading edge and the trailing edge.
In order to remove the skew, a leading edge of a sheet, provided at a nip region of a downstream roll pair, is stopped while an upstream roll pair continues to advance the sheet to thereby form a buckle. The buckle ensures that the leading edge straightens out against the nip region of the downstream roll pair. The substrate is then pulled straight through the nip after the buckle has been formed and the skew has been removed.
Conventional reproduction systems typically include three supply paths that are used to supply sheets to a print engine. The supply paths typically include (1) a path that transports sheets from a sheet supply tray that stores a plurality of sheets within the reproduction system, (2) a path that transports sheets from a multi-purpose tray that stores sheets on a tray attached to an outside of the reproduction system and (3) a path that returns printed sheets to the print engine so that double sided printing can be performed.
Typically, there are separate deskew systems for each path in order to correct deskew errors due to paper skew, process edge deskew errors in the process direction and/or lateral edge deskew errors. Each of the deskew systems include a one-sided buckle chamber that is bordered on one side with the deskew nip and on another side with a common transport roller with a sufficient nip load. After a firm grip has been achieved by the common transport roller, a buckle is formed on only one side of the buckle chamber.
Also, in most conventional deskew systems used in various reproduction systems, the types of substrates being transported usually do not vary much. That is, many systems typically encounter only a limited number of different substrate types, such as basic draft sheet stock of a certain weight in basic sizes such as A4 or 8.5×11 inches. A typical deskew system is designed to transport, for example, 20 lb. bond sheet stock (roughly 75 grams/m2 or GSM). Occasionally, higher quality bond paper of a slightly higher weight, such as 24 lb. bond (roughly 90 GSM) or 28 lb. bond (roughly 105 GSM) sheet stock is used. In conventional deskew systems, these sheets are transported using the same drive profiles. That is, the drive control parameters are fixed (i.e., set irregardless of the weight of the sheet being used).
In the United States, paper weight is expressed as pounds per 500 sheet ream of uncut C-size paper (4×letters size). As such, a cut ream of 20 pound letter paper (500 sheets of 8.5×11) would weigh 5 pounds. Because each type of paper has a different “basis size”, it is often confusing to talk in terms of the U.S. pound weight system. Instead, it is much more convenient to express paper “weight” as mass per unit area as in the ISO (metric) system in which the weight of paper is given in grams per square meter (GSM). For example, 20 pound bond letter stock corresponds to roughly 75 GSM, 24 pound bond letter stock corresponds to roughly 90 GSM, and 28 pound bond letter stock corresponds to roughly 105 GSM. 20 pound Bristol board on the other hand, which has a different basis size, corresponds to roughly 44 GSM. Other known substrates can have substantially higher GSM, some over 300 GSM.